Autocatalytic gold plating solutions

ABSTRACT

IMPROVED ELECTROLESS GOLD PLATING BATHS ARE PROVIDED BY FORMULATING IN AQUEOUS ALKALINE MEDIA, A WATER SOLUBLE GOLD SALT; A COMPLEXING AGENT FOR GOLD, A WATER SOLUBLE BOROHYDRIDE OR AMINE BORANE REDUCING AGENT FOR GOLD, AND A SMALL EFFECTIVE STABILIZING AMOUNT OF A CYANIDE COMPOUND IN AN AMOUNT OF BETWEEN ABOUT 5 MICROGRAMS AND 500 MILLIGRAMS.

United States Patent 3,589,916 AUTOCATALYTIC GOLD PLATING SOLUTIONS John F. McCormack, Roslyn Heights, N.Y., assignor to Photocircuits Corporation, Glen Cove, NY.

No Drawing. Continuation-impart of abandoned application Ser. No. 768,575, Aug. 20, 1968, which is a continuation-in-part of abandoned application Ser. No. 377,509, June 24, 1964. This application Nov. 14, 1969, Ser. No. 877,052

Int. Cl. C230 3/02 US. Cl. 106-1 9 Claims ABSTRACT OF THE DISCLOSURE Improved electroless gold plating baths are provided by formulating in aqueous alkaline media, a water soluble gold salt; a complexing agent for gold, a water soluble borohydride or amine borane reducing agent for gold, and a small elfective stabilizing amount of a cyanide compound in an amount of between about micrograms and 500 milligrams.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending application, Ser. No. 768,575, filed Aug. 20, 1968, now abandoned, which, in turn is a continuation of Ser. No. 377,509, filed June 24, 1964, now abandoned.

The present invention relates to a new and improved process for the autocatalytic deposition of gold, and to novel and improved autocatalytic plating solutions capable of electrolessly depositing gold on catalytic surfaces in contact therewith.

The present invention has for its object the provision of novel and improved processes and compositions for the electroless plating or deposition of bright layers of gold on various surfaces.

A further object of this invention is the provisions of novel and improved electroless plating baths for the deposition of gold which are simple to use, operate with certainty, have improved stability against spontaneous decomposition and which are economical.

The invention consists in the novel steps, processes, compositions and improvements herein shown and described.

Chemical plating solutions for continuously depositing gold by autocatalytic chemical reduction of gold ions dissolved in the solutions and in contact with a catalytic surface of an article to be plated are well known. Such solutions, which do not utilize electricity, are sometimes referred to in the art as electroless metal plating solutions, to distinguish them from electroplating solutions, which do require the use of electricity. Electroless metal deposition is also to be distinguished from displacement metal plating of the type described in Metals Finishing Guide Book, 27th Edition, 1959, page 469 et seq., and metal mirroring procedures, wherein the metal plating desired and achieved is only a few millionths of an inch in thickness.

DESCRIPTION OF THE INVENTION According to this invention, there are provided autocatalytic solutions for depositing gold electrolessly which comprise an aqueous alkaline solution of a water soluble salt of gold, a borohydride or amine borane reducing agent, a complexing agent for gold and a small, stabilizing amount of a cyanide compound.

Electroless gold baths of the type known in the prior art until now have a tendency to decompose after rela- Patented June 29, 1971 tively short periods of time. Further, such baths tend to produce deposits of gold which are brittle and exhibit poor ductility.

To enhance the useful life of the bath and improve the ductility of the deposit, it has now been found that water soluble cyanide compounds may be added to the bath. The amount to be added appears to be somewhat critical; especially the upper limit is critical.

Among the water soluble cyanide compoundsmay be mentioned alkali cyanides, such as sodium and potassium cyanide, and nitriles, such as alpha-hydroxynitriles, e.g., glycolnitrile and lacton'itrile, and a-hydroxyisobutyronitrile. -In general, the cyanide compound will be added in an amount of between about 5 micrograms and 500 milligrams per liter. Care should be used in adding the cyanide compound to insure that an excess, i.e., more than 500 mg./l., is not employed. When too much cyanide is used, the present plating baths will not function, i.e., they will slow down to the point where' they will not .autocataly-tically deposit gold from solution when in contact with a catalytic surface.

The gold deposits produced by autocatalytic baths containing cyanide compounds of the type described are bright and ductile. Additionally, as has been mentioned, the baths are more stable and have a longer useful life than the baths in which cyanide is not used.

The reducing agent may consist of any water soluble borohydride or amine borane having a good degree of solubility and stability in aqueous solutions. Sodium and potassium borohydrides are preferred. Also may be mentioned substituted borohydrides, such as sodium trimethoxy borohydride, NaB(OCH H. Useful too are the amine boranes, such as monoand di-lower alkyl, e.g., up to C alkyl, amine boranes, e.g., isopropyl amine borane and dirnethyl amine borane.

In order to prevent spontaneous decomposition of the baths, the plating solution described herein will ordinarily be operated at high pH, preferably at between pH 10 and 14. It is preferred to use an alkali metal hydroxide, e.g., sodium or potassium hydroxide to atain this pH.

When sodium borohydride is used as the complexing agent, the pH will ordinarily be in the vicinity of 13-14. With potassium borohydride, the pH will generally be maintained between about 11 and 12. With the amine boranes, the pH will be between about 10 and 14.

In preparing the solution, the water soluble sulfates chlorides, acetates, or nitrates of gold will ordinarily be utilized as the water soluble salts. Other water soluble salts may be utilized, however, the choice being primarily a question of economics.

The sequestering or complexing agent used has to form a sufficiently strong complex with the gold ions to pre vent the precipitation of metallic gold or salts of gold. Further, the complexing agent must be capable of forming a complex with gold ions which is soluble in the plating solution, and also which is sufficiently stable so that it will not react with the reducing agent in the main body of the plating solution, but only at or in the vicinity of the catalytic surface to be plated.

The complexing or sequestering agents suitable for use include ammonia and organic complex-forming agents containing one or more of the following functional groups: primary amino group (-NH), secondary amino group NH), tertiary amino group N-), imino group (=NH), carboxy group (COOH), and hydroxy group (OH). Among such agents may be mentioned etheylene diamine, diethylene triamine, triethylene tetramine, ethylenediamine tetraacetic acid, citric acid, tartaric acid, 1,3-propane diamine, and ammonia. Rochelle salts can be used. Related polyamines and N-carboxymethyl derivatives thereof may also be used.

Generally, Rochelle salts are preferred for gold baths.

In preparing the electroless metal deposition baths, it is desirable to combine the bath ingredients in such a manner as to avoid reaction between the soluble metal salt and the reducing agent.

It is therefore preferred to first add the complexing agent to the aqueous solution of the metal salt to form the water soluble complex of the gold cations. If the reducing agent is added before the metal complex is formed, there will be a tendency for it to react instantaneously with the gold salt to precipitate the metal or a salt of the metal. After forming the metal complex, the pH is adjusted to the proper value before adding the reducing agent. The reducing agent will ordinarily be added in the form of an alkaline aqueous solution.

The catalytic surfaces which may be plated with the baths of this invention include metals such as nickel, cobalt, iron, steel, palladium, platinum, copper, brass, manganese, chromium, molybdenum, tungsten, titanium, tin or silver. All such metals are catalytic to the reduction of the metal cations dissolved in the solution described. In some cases, it is useful to sensitize the substrate by treatments well known to those skilled in the art.

Materials such as glass, ceramic, and various plastics are, in general, non-catalytic. However, the surfaces of such non-catalytic materials can be rendered catalytic by producing a film of particles of one of the catalytic materials thereon. This can be accomplished by a variety of techniques known to those skilled in the art. One suitable sensitization procedure involves dipping non-catalytic materials in an acidic solution of stannous chloride, Washing with Water, and then contacting the material with an acidic solution of a precious metal salt, e.g., palladium chloride. A mono-layer of precious metal is thus produced, which mono-layer is catalytic to reduction of the metal ions in the electroless metal plating solutions.

Alternatively, such materials as glass, ceramic, and plastic may be sensitized or rendered catalytic by treatment with an acidic aqueous solution containing, in combination, stannous tin ions and precious metal ions.

A further sensitization procedure involves adhering to the normally non-catalytic surface, finely divided particles of metals or metal oxides which are catalytic to electroless metal solutions. 7

The term catalytic surface as used herein refers to the surface of any article composed of the catalytic material described hereinabove or covered therewith or to the surface of a non-catalytic material which has been sensitized by producing a film of particles of said catalytic materials on its surface.

The baths of this invention will deposit gold electrolessly on the catalytic surfaces of metals and non-metals, such as paper, glass, ceramic, synthetic resins and plastics, including but not limited to silicones, phenolics, alkyds, epoxies, styrenes, acrylics, vinyl chlorides, nylon, mylar, acrylonitrile-butadiene-styrene, and the like.

The quantities of the various ingredients in the baths of this invention are subject to wide variation. Typically, however, the bath constituents will be as follows:

Water soluble gold salt: 0.002 to 0.60 moles/liter Reducing agent: 0.0002 to 2.5 moles/liter Complexing agent: 0.7 to 10 times the moles of metal salt Cyanide stabilizer: micrograms to 500 milligrams/liter Alkali metal hydroxide: sufficient to provide a pH of to 14 The amount of sequestering agent to be added to the plating solution depends upon the nature of the sequestering agent and the amount of the gold salt present in the bath. In alkaline solutions, the preferred ratio of the gold salt to complexing agent lies between about 1:1 and 1:10. A small excess of the sequestering agent, based upon the gold salt, generally is advantageous.

With the baths disclosed, gold deposition occurs autocatalytically at a uniform rate wherever there is contact between the catalytic surface being plated and the plating solution. There is no substantial variation in the plate thickness even for the most complicated shapes. Thus, gold may be uniformly deposited in recesses, as well as on exposed parts of the object being plated, and there is no build-up of coating at points or edges. These conditions are difficult or impossible to achieve by electroplating. Becasuse of the uniform deposition achieved, the plating process described is particularly suitable for plating objects of irregular or complicated shapes which are diflicult or impossible to metallize by conventional techniques.

The process of this invention permits gold to be deposited on practically any conceivable substratum and to practically any thickness desired.

It should be understood that as the baths are used up in plating, the gold salt, and the reducing agent may be replenished from time to time, and also that it may be advisable to monitor the pH, and the solution ingredients, and to adjust them to their optimum value as the bath is used.

For best results, surfactants in an amount of less than about 5 grams per liter are added to the baths disclosed herein. Typical of suitable surfactants are organic phosphate esters, and oxyethylated sodium salts. Such surfactants may be obtained under the trade names Gefac RE 610 and Triton QS-lS, respectively.

In using the autocatalytic or electroless gold solutions the surface to be plated must be free of grease and other contaminating material.

Where a non-metallic surface is to be plated, the surface areas to receive the deposit should first be sensitized as described hereinabove.

Where a metal surface, such as stainless steel, is to be treated, it should be degreased, and then treated with acid, such as hydrochloric or phosphoric acid, to free the surface of oxides.

Following pre-treatment and/or sensitization, the surface to be plated is immersed in the autocatalytic bath, and permitted to remain in the bath until a gold deposit of the desired thickness has been built up.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A typical plating bath made in accordance with teachings contained herein is given below:

EXAMPLE 1 Gold chloride hydrochloride trihydrate (AuCl -HCl- 3H 0) 0.01 moles/ liter Sodium potassium tartrate: 0.014 moles liter Dimethyl amine borane: 0.013 moles/liter Sodium cyanide: 400 milligrams/liter Water: q.s.a.d.

pH (adjusted with NaOH): 13 Temperature: 60 C.

Metallized surfaces produced by utilizing the autocatalytic gold deposition solutions of this invention are useful as ornamental designs, markings, and the like. Similarly, using these baths gold may be deposited in predetermined patterns on substrata and serve as electrical conductors. Electrically conductive circuit patterns, e.g., printed circuits, may thus be selectively plated on the activated areas of an inexpensive sheet of a wide variety of insulating material.

The invention in its broader aspects is not limited to the specific steps, processes and compositions described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. An aqueous alkaline chemical plating solution capable of autocatalytically depositing gold on a substratum in contact therewith, comprising, in combination, water; a water soluble salt of gold in an amount of from about 0.002 to 0.60 mole per liter; a complexing agent for said gold wherein said complexing agent is a member selected from the group consisting of ammonia and compounds containing at least one of an amine group, carboxy group and hydroxy group, said complexing agent forming a stable, water soluble complex with gold in an amount of between 0.7 and times the moles of gold salt; as a reducing agent, a member selected from the group consisting of water soluble alkali metal borohydrides and water soluble amine boranes in an amount of between 0.0002 and 2.5 moles per liter and a small effective stabilizing amount of a water soluble cyanide compound in an amount of between about 5 micrograms and 500 milligrams per liter and wherein the pH is between about 10 and 14.

2. A solution as defined in claim 1 wherein the pH is between about 12.0 to 14.0.

3. A process for electrolessly depositing gold on a catalytic surface 'which comprises contacting the surface with an aqueous alkaline solution comprising a Water soluble salt of gold in an amount of from about 0.002 to 0.60 mole per liter; a complexing agent for said gold wherein said complexing agent is a member selected from the group consisting of ammonia and compounds containing at least one of an amine group, carboxy group and hydroxy group, said complexing agent forming a stable water soluble complex with gold in an amount of between about 0.7 and 10 times the moles of gold salt; as a reducing agent, a member selected from the group consisting of a water soluble alkali metal borohydride and a water soluble amine borane in an amount of between about 0.0002 and 2.5 moles per liter and a small effective stabilizing amount of a water soluble cyanide compound in an amount of between about 5 micrograms per liter and 500 milligrams per liter and wherein the pH is between about 10 and 14.

4. A process as defined in claim 3 wherein the pH of the solution is maintained between about 12 and 14.

5. A process as defined in claim 3 wherein the cyanide compound is a member selected from the group consisting of water soluble inorganic cyanides and water soluble organic nitriles.

6. A process as defined in claim 5 wherein said cyanide compound is selected from the group consisting of potassium cyanide, sodium cyanide and alpha-hydroxynitriles and wherein said compound is present in an amount low enough to permit the bath to function.

7. An aqueous alkaline electroless gold plating bath solution comprising water, gold ions in the form of a water soluble gold salt in an amount of between about 0.002 to 0.60 mole per liter, a complexing agent for said gold ions which is a member selected from the group consisting of ammonia and compounds containing at least one of an amine group, carboxy group and hydroxy group, said complexing agent being capable of forming a stable water soluble complex with said gold ions in an amount of between about 0.7 and 10 times the moles of gold salt, a reducing agent for said gold ions which is a member selected from the group consisting of sodium borohydride, potassium borohydride, sodium trimethoxy borohydride, isopropyl amine borane and dimethyl amine borane, said reducing agent being present in an amount of between about 0.0002 and 2.5 moles per liter and a small elfective stabilizing amount of a compound which is a member selected from the group consisting of water soluble inorganic cyanide compounds and water soluble organic nitrile compounds, said compounds being present in an amount of about 5 micrograms to 500 milligrams per liter and wherein the pH is between 10 and 14.

8. A solution as defined in claim 7 wherein said reducing agent is dimethyl amine borane.

9. A solution as defined in claim 7 wherein the stabilizing compound is a member selected from the group consisting of potassium cyanide, sodium cyanide and alphahydroxynitriles.

References Cited UNITED STATES PATENTS 3,300,328 l/1967 Luce 1l7130X 3,468,676 9/1969 Schneble et al 106-1 LORENZO B. HAYES, Primary Examiner U.S. Cl. X.R.

@2 33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION D d June 29, 1971 John F. McCormack Patent No.

Inventofla) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 42, "complexing"- should read reducing Column 2, line 67, "etheylene" should read ethylene Column 4, line 10, "Becasuse" should read Because Column 4, line 28, "Gefac" should read Gafac Signed and sealed this 28th day of November 1972.

(SEAL) Attast:

ROBERT GOTTSCHALK 

